As senescence occurs, chlorophyll production grinds to a halt, and the existing chlorophyll gets broken down. This causes the natural xanthophylls and carotenoids (yellows and oranges) also present in the leaf to be the dominant colors. In some types, especially oaks, there are also tannins present in large quantities that produce browns. But there are also new compounds produced as by-products of the chorophyll destruction and the general decay of the leaf. These are responsible for the vibrant red and purple colors (in particular, anthocyanins) that you see in leaves, especially in the later stages of autumn.

Source: names of compounds were checked at http://ncnatural.com/wildflwr/fall/science.html